The known methods of forming a stitch are basically of two types, chain stitch and lock stitch. The single thread chain stitch is formed from one upper thread, while the double thread chain stitch utilizes an upper and a lower thread. The stitches in the single chain stitch are formed by inserting each upper needle thread loop through the preceding needle loop and retaining the loop open to enable insertion of the subsequent loop. In the double thread chain stitch, the upper needle thread loops are passed through the material and are interlaced with the loops of the under thread. Inasmuch as it is not necessary to pass a supply of thread completely through the loop in a chain stitch, it is possible to utilize a very large supply of thread. A disadvantage of the chain stitch resides in either a skipped stitch during formation or the severing of a stitch which will result in the unraveling of a number of the stitches.
The lock stitch is formed from an upper or needle thread and a lower or bobbin thread. A loop of the needle thread is passed through the material, and the entire supply of the lower thread is passed through the loop, leaving a single strand extending through the loop. The supply of lower thread is of necessity limited by the fact that it has to be passed through the needle thread loop. This creates a disadvantage in that the changing of the thread supply from time to time creates down time on the machine. However, severing of one stitch does not involve the loss by unchaining of a number of stitches.
It has been proposed to form the lock stitch from a single needle thread by dividing the thread into two parts on the first needle penetration. The first part is the upper needle thread and the second part is the lower thread. On subsequent penetrations of the needle, the lower thread is passed through the loop formed in the upper thread. The disadvantage of this stitch is period of time required to wind the lower part on the bobbin or the slow speed at which the machine must operate in order to ascertain that the under thread is in its proper position and under correct tension at all times.